*3.5. Tribological Tests*

Figure 10 demonstrates the processes occurring in the contact zone of the tool with the workpiece. At room temperature (Figure 10a), the curves of change in the coefficient of friction (COF) of various tool materials in contact with a steel pin have a classical trend, and the coefficient of friction throughout the entire path demonstrates a stable character with small peaks at the initial stage of the tests for all investigated material samples. At the same time, the coefficient of friction is different for different materials: the COF has a maximum value of 0.75 for a sample made of HS6-5-2C (100% HSS), while the COF remains at the level of 0.45–0.6 for samples of CPHSS after running-in and stabilization of the contact interaction. The smallest COF (0.45) among the experimental CPHSS materials under study is shown by a sample containing 80% HSS and 20% TiCN, and the largest (0.6) by a sample containing 80% HSS and 20% TiC. The results of tribological tests under high-temperature heating conditions (Figure 10b) have a completely different form, i.e., a level similar to the thermal effect that tool contact pads undergo in the real operating conditions. The curves of the friction coefficient change for a number of the samples under study have sharp peaks and a pronounced unstable character. It is typical of a sample made of HS6-5-2C material (the nature of the curves was repeatedly checked to eliminate errors), for which COF "jumps" from 0.6 to 1.1 are observed over 70 m of the friction path, and then relative stabilization is observed with a gradual increase (COF is 0.9 at 200 m). A sample containing 80% HSS and 20% TiC shows a somewhat stable character and for the highest COF value of all CPHSS samples, the COF develops abruptly, and its values range from 0.5 to 0.75 at a distance of 40–140 m; a pronounced stabilization of the friction conditions is observed, and the average COF value is 0.65 at a distance of 150 m. The most favorable friction conditions are demonstrated by a sample containing 80% HSS and 20% TiCN:


**Figure 10.** Dependences of the coefficient of friction (COF) of various tool materials on the friction path in contact with a steel pin without heating (**a**) and when heated at +600 ◦C (**b**): 100% HSS (1); 80% HSS, 20% TiC (2); 80% HSS, 15% TiC, 5% Al2O3 (3); 80% HSS, 20% TiCN (4).

COF behavior under high-temperature exposure (Figure 10) is directly related to the structural features of the tool materials. The primary structural component is tungsten and molybdenum carbide Fe3(W, Mo)3C for 100% HSS. The available results of the behavior of various grades of HSS when studying at high-temperature frictional contact with lowcarbon steels [46,47,59] show that under thermal action with an increase in the number

of test cycles, there is gradual coagulation and shape change of carbides from spherical to drop-shaped and multifaceted, the crystal lattice of the material changes, and a layer consisting of martensite and austenite forms on the surface. At the same time, a decrease in the hardness and strength of the HSS surface layer is observed. The stability of the crystal lattice significantly affects the coefficient of friction, and the decrease in the tool steel hardness significantly increases the friction force adhesive component.

A different mechanism takes place at high-temperature frictional contact of CPHSS samples. The refractory compounds TiC, TiCN, and Al2O3 present in the samples' structures upon heating contribute to an increase in the density of defects, and a weakening of interatomic bonds occurs, which improves the oxygen access to the surface layer, and the rate of formation of very hard oxide films with metals increases (in particular, titanium begins to interact with oxygen at temperatures of 600 ◦C and above). Three stages of contact interaction (Figure 3) explain the described mechanism well. In addition, with an increase in temperature, an increase in the plasticity of refractory compounds is observed. Their lubricating effect is manifested. The specific surface energy in the surface layer decreases, and the work expended on surface deformation decreases. This explains the decrease in the COF for the CPHSS samples. Similar phenomena were observed by the authors of other works when studying the frictional interaction of various tool materials with carbides and nitrides of refractory metals-based coatings [12–14,16,22,37].

A comparison of the operational and tribological characteristics based on the test results of various tool materials described above show strong correlations, where the maximum wear rate during cutting and the worst tribological characteristics during hightemperature testing were shown by a sample of HS6-5-2C powder, and the best performance indicators and tribological properties by the sample containing 80% HSS and 20% TiCN. There is no doubt that the presence of refractory compounds of the TiCN type in the structure of the tool material can provide structural adaptation (self-organization) of the surface layer under external heat–power action due to the formation of secondary structures with thermodynamic stability and improved lubricity.

The most informative is spectroscopic methods to identify and prove the formation of secondary phases. In this case, it is important to analyze the thin surface layer of the tool's contact pads and the workpiece being processed. Figure 11 shows the results of secondary ion mass spectrometry of worn-out contact pads of the tool made of CPHSS material containing 80% HSS and 20% TiCN (Figure 11a) and auger electron spectroscopy of the workpiece's processed surface (Figure 11b) after specific time intervals of the intermittent cutting process. A CPHSS sample that showed maximum wear resistance in service was deliberately analyzed.

The data obtained show that in machining, the tool material containing refractory inclusions of TiCN interacts with environmental components and elements that make up the workpiece to be processed, forming new (secondary) phases. In turn, the chemical composition of the surface layer of the workpiece also undergoes a noticeable transformation (Figure 11).

Spectral analysis shows very different compositions of the surface of the tool contact pads in the initial period of cutting (during running in) and in the time interval of steadystate wear after 20 min of operation (Figure 11a). Spectral analysis revealed the highest intensity of the spectrum of the metastable compound based on TiC2 (titanium dicarbide) and less intense spectra of TiN and TiO (in decreasing order of intensity) after 5 min of running in. The wear rate of the contact pads stabilizes with the further operation of the tool under the influence of heat and power loads of the cutting process and the external environment's effect. Spectrometry shows a sharp decrease in the intensity of the TiC2 spectrum in comparison with the running-in zone, and one can observe the decomposition of titanium dicarbide and its transition to the oxygen-containing TiO phase (maximum spectrum intensity) and partially to TiN, since the intensity of its spectrum increases markedly in comparison with the running-in zone.

It is precisely thin surface films based on secondary phases such as thermally stable compounds of titanium with oxygen and nitrogen, which have good lubricity, that significantly reduce the intensity of the adhesive and frictional interaction of the tool and the workpiece during milling and provide the previously established (Figure 8) twofold increase in resistance and improve the workpiece surface quality.

**Figure 11.** Results of secondary ion mass spectrometry of worn tool pads made of CPHSS containing 80% HSS and 20% TiCN (**a**) and auger electron spectroscopy of the surface of a machined workpiece made of 41CrS4 steel (**b**) after 5 and 20 min of operation in interrupted machining at *V* = 68 m/min, *f* = 0.15 mm/rev, *t* = 2 mm.
